Standard SNMP [Simple Network Management Protocol] is defined by the IAB [Internet Architecture Board]. It requires the use of a prescribed database structure—called a Management Information Base, or MIB—to interface with objects. RMON defines certain extensions to the basic SNMP MIB. The three basic specifications relating to SNMP are:                “Structure and Identification of Management Information for TCP/IP-based “networks” (RFC 1155), defines how managed objects are characterised in the MIB.        “Management Information Base for Network Management of TCP/IP-based Internets” (MIB-II: RFC 1213) defines the managed objects that can be contained in a MIB.        “Simple Network Management Protocol” (RFC 1157) defines the protocol used to manage these objects.        
[See: “SNMP, SNMPv2, SNMPv3, and RMON 1 and 2” by William Stalling, Addison Wesley, 3rd Edition, 1999]
A TCP/IP network managed using SNMP employs at least one management station [NMS] that maintains a database of information extracted from the MIBs of all managed objects within the network. Information collection or extraction is mediated by network management agents (implemented in software) at key platforms, hosts, bridges, hubs, routers, which are able to interrogate object MIBs and to receive unsolicited information therefrom. It is, of course, necessary for every management station, agent and object in this system to support the common IP protocol and, preferably, to have a standard IP address.
Because of the substantial cost involved, IP is not implemented on many simple devices that are part of a larger system that is connected to a TCP/IP network. These devices are therefore not network objects capable of being managed by the NMS using SNMP; they do not have IP addresses and are essentially invisible to the NMS. Often, such devices are inter-connected by a proprietary monitoring or controlling network that does not use TCP/IP protocols, the private network and its devices often being referred to as ‘legacy’ systems and devices. In other cases, the lack of an IP interface for a ‘legacy’ device is a consequence of the age of the device; that is, it probably was installed and commissioned before SNMP became ubiquitous. While it is possible to provide a SNMP interface for a legacy device, it would effectively involve the provision of a dedicated computer for each device in order to set-up TCP/IP, establish an IP address and a MIB for that device and to monitor the state of the device.
Moreover, for commercial reasons rather than cost considerations, IP is not implemented on many modern proprietary devices. For example, proprietary multiplexers, demultiplexers, transmitters and receivers employed in telecommunications links, and perhaps containing many thousands of individual devices, are often not implemented as standard network objects. The manufacturers of such complex systems often prefer to connect them via proxy agents to TCP/IP networks running SNMP, the proxy agent software and associated MIBs being proprietary and, often, non-standard. These proxy agents are connected to the devices of the system using proprietary protocols, not IP. Thus such devices are not themselves network objects. In some cases, the proxy agent acts primarily as a protocol converter. For example, Siemens manages devices in some of its SDH [Synchronous Digital Hierarchy] systems using a Q3 protocol in conjunction with its proprietary EMOS device manager, and the University College London has written a protocol converter to interface Q3 with SNMP that can be implemented as the core of a TCP/IP proxy agent. Other examples are CORBA and CMIP that will be known to those skilled in the art.
A major drawback of the proxy agent approach is that that the MIB of the proxy agent tends to be very complex (since it must cover all devices in the proprietary system) and it cannot be readily scaled or modified to take account of the addition, removal or change of devices. This results in inflexibility and, often, the failure of the MIB to accurately reflect the status of the devices or system being monitored via SNMP. The proxy agent, of course, necessarily has a single IP address (corresponding to its single MIB).
In yet other cases, where the network elements to be integrated under a common NMS are private sub-networks each using its own set of IP addresses, integration is impossible because pre-assigned IP addresses in the private IP have already been assigned by the NMS to existing elements of the main network. In other words, connection of such a private network would result in ambiguity and confusion caused by the conflicting or overlapping addresses or address fields. This may require complete reconfiguration of the private network or the reassignment of addresses therein. Such reconfiguration can be expensive and cause considerable inconvenience to existing users of the private network concerned.